Hi hettie,
As tony1851 and cjard have indicated, the key point is that because the proposed extension would be attached to the existing extension, it would be necessary to assess the
combined extension against several of the limitations and conditions of permitted development legislation. For your situation, this combined extension wouldn't be permitted development, because its maximum height of 4.3m would be contrary to limitations A.1(e)(ii) and A.1(h)(i) of Part 1 Class A.
From a technical point of view, the reason it would be necessary to assess the combined extension is because many of the limitations and conditions of Part 1 Class A use the phrase
"the enlarged part of the dwellinghouse". Overall, evidence supports the interpretation that this phrase not only applies to the proposed extension, but also includes any existing extension to which the proposed extension would be attached.
For reference, out of the appeal decisions that I've seen that have dealt with the above issue, the majority (i.e. 8 appeal decisions) have supported the above interpretation whereas the minority (i.e. 4 appeal decisions) have contradicted the above interpretation. However, more significant is the fact that the examples on pages 27 and 28 of the DCLG "Technical Guidance" document (indirectly) support the above interpretation:
http://www.planningportal.gov.uk/uploads/100806_PDforhouseholders_TechnicalGuidance.pdf
For info, the above interpretation (i.e. that it's necessary to assess the combined structure) doesn't depend on how the existing extension became lawful. For example, maybe the existing extension was PD under the previous version of Part 1 of the GPDO, or maybe it was granted PP by the Council, or maybe it was erected unlawfully and became lawful via the 4-year rule. Regardless of how the existing extension became lawful, if the proposed extension would be attached to the existing extension then the combined extension would form
"the enlarged part of the dwellinghouse".
Thanks,
Steve